Junction transistor



April 1961 w. SHOCKLEY 2,980,830

JUNCTION TRANSISTOR Filed Aug. 22, 1956 5 Sheets-Sheet 1 F'llEi L INVENTOR. x- W/W/am Shock/6g April 18, I961 w. SHOCKLEY 2,930,830

JUNCTION TRANSISTOR Filed Aug. 22, 1956 5 Sheets--Sheet 2 INVENTOR. Vl /'///l9m J770ck/eg Aprll 18, 196] w. SHOCKLEY JUNCTION TRANSISTOR 5 Sheets-Sheet 3 Filed Aug. 22, 1956 INVENTOR. MAW/am .f/mck/eg M ll M ATTORNEY:

April 1961 w. SHOCKLEY 2,980,830

JUNCTION TRANSISTOR Filed Aug. 22, 1956 5 Sheets-Sheet 4 FlEi. LE 1 IE 1 1 m Will/Ill,

INVENTOR. Why/am JP'wck/eg BY 7* 7 7'7'OP/VE Kr April 18, 1961 w. SHOCKLEY 2,980,830

JUNCTION TRANSISTOR Filed Aug. 22. 1956 5 Sheets-Shet 5 INVENTOR. M ////'am J7; ads/sq ,a juncti ans r;

pacity.

This invention relates generally to a junction transistor and more particularly to a junction transistor having relatively low base resistance.

Junction transistors are constructed by the diffusion .method, the grown junction method and the rate grown junction method. The difiusion method is generally employed to construct transistors having relatively high alpha cut-off frequencies. These transistors include a collector body which is a relatively massive block having anohmic contact formed on one face. The base' layer is a relatively thin'layer which is in the order of 1 mil wide by 6 mils long for highfrequency transistors. I a Qhmic contact is made along one edge of the base layer.

The emitter body is likewise a relatively thin layer which overlies the base layer. Ohmic contact is made on the upper face. t i

As is Well-known, the alpha cut-off frequency of the base-layer in a junction transistor is dependent upon the thickness. The gain of the transistor and its high ,fre-

quency behavior depends also on the resist-ance of the base layer. 1f the length and width of the base layer are held constant and the thickness decreased, the resistance Unit-ed States Pate f i? per unit length is increased Since both resistance and thickness afiect the alpha cut-oif frequency, nothing is gained by reducing the thickness and holding theother dimensions constant. However, the resistance may be decreased by making the base layer relatively narrow. This leads to an extremclytsmall structure since, in practice, it rnay be necessary to reduce the "width of the base layer to a fraction of a thousandth of an'inch in order to take advanta'geof attainable base layer thicknesses. .Such

small structures are diflicult t o' fabricate, and the power capacity of the transistors is limited. i

One method which has been proposed for reducing the base resistance and yet maintain base dimensionsis to formta grid-like metallic structure in theb'as e. One disadvantage with this method is that it necessitates placing metal electrodes in the body of the semiconductor within the base layer and this is impractical for very thin base layers. l ltfis a general ,object ot' the present inventionto profor constructing the same. i I if I It is another object of the present invention to provide :a .transistorjstructure in'whichithe b'ase'layer resistance vide an improvedv junction transistor strncture and method i; Itis another v bject of the present invention to provide a iuns Hams e leavin s at r l i al i frequencies. t i t i lt isra ncth el objectof the present invention to pIQ ness;

A 2,980,830 Cg e ted APP- 8a It is still another object of the present invention to provide a transistor having a base layer which coiisists of a web and ribstructure.

It is a further object of the present inventionto provide a transistor having a base layer which comprises a web and rib structure and in which the'ribshavfe' a higher carrier concentration.

It is another object of the present invention to facilitate and improve connections to a restricted zone in aseniiconductive body. i i

It is another object of the present invention to provide a method for the fabrication of a junction transistor of the above type. a

The invention possesses other objects and features of advantage, some of. which, with theforegoingwillbe set forth in the following description of the invention. It is to .be understood, of course, that the inventioni's not to be limited to the particular disclosure of species .offthe invention, as other variant embodiments'may be adopted within the scope of the appended claims.

7 Referring to the drawing:

Figure 1 shows diagrammatically a junction transistor illustrative of one embodiment ofthe invention; Figure 2 is a sectional view taken along the line l2 ofFigure 1;

Figure 3 is an equivalent small signal circuitfor-a junction transistor;

Figure 4 is an enlarged view of a suitable collector region for a junction transistor of this inventionj' and rib of Figure 1;

Figure 6 isa curve showing the chemical charge density along the lines AA and B- B.of Figure .5; i Figures 7AH show a suitable method for fabricating a junction transistor haying'a bae layer of varyingthibk- Figures 8A-I show another embodiment of the invention and method for fabricating the same; tlie'figur includes curves showing the chemical charge density lat various stages of construction; v i Figure;9 shows another embodiment of the invention and method for forming the base region of the same;

Figure 10 shows still another embodiment'of the inventiori'and method for constructing the same;

- Figure 11 shows a vacuum chamber for applying suitable coatings to theeinbodiment of Figure 10;

Figure "12 is atop view of a transistor constructed'in accordance-with Figures 1 0 and 111 Figure '13 is. a sectional viewtaken along the lines Figure 14 shows still another embodiment of the 'invention comprising a small area junctiontransistor;

Figure 15 is a view taken along the'lihe' '1 5-"*15 of Figure 16 isa view taken along the lines1,6-'16, of

Figure 14. i V Referring to Figures 1 and 2, a junction transistor having a high conductivity base region of varying'thickiiess is shown. The transistor comprises a relatively massive collector block 0 of s'emi conductive r'riaterial arena conductivity type, for example p-type gen'naniu'm'br silicm, withohmic contact 11, base laykergbjofseriti-coir- ,ductive material of opposite conductivity type, for example n-type germanium or ,silicon, with ohmic contact 12 alO lg one edge ,thereof, and an ifimitter region 122' also of suitable semi-conductive material of. the' same con- ,ductivity type as the collector (pt-typeqss manium r silicon),,wi

.ohmiocontactdTs. v 0 rticularly to Figure 2, -:th Webs" or pl ates' lo gl layin 111 Refe :portion of the base between the ribs. between base and emitter that enters the ribs will not contribute appreciably to the over-all alpha of the transistor I when the operating frequency lies between the alpha cutany small area of web has a relatively low resistance path through rib material to the base metallic contact comaeaopao ff' pared to the path it would have if the base layer were of uniform thickness and composed entirely of web. In

' efiect, the rib material connects the metal contact, through a relatively low resistance path, to the edges of relatively narrow areas of web material, thus reducing the resistance,

from any 'part of the web to the base contact.

As a consequence the average distance of all parts of the web from the ribs is substantially less than the average distance of all parts of the web from the base contact.

Due to the largerthickness of the ribs, the alpha cutoff frequency for them is much lower than for the web Current flowing oii frequency for the web region and the alpha cut-off frequency for the rib region. As will presently be described, however, the injected current entering the rib region is much smaller than that entering the web region;

therefore, the reduction of alpha due to ribs is much smaller than the proportional area of the base occupied by ribs. From quantitative considerations of the relative "importance" of the alpha value and the value of the base resistance, it can readily be shown that transistors of improved performancewill result with a ribbed base ,structure.

The importance of base resistance in controlling the gain of junction transistors is well-known. A discussion given in considerable detail will be found in an article ..entitled A Junction Transistor Tetrode for High Frejquency Use by R. L. Wallace, In, L. G. Schimpf, Proceedings of the IRE, volume 40, page 1395, November I 1952. An appreciation of some of the main features of a the foregoing, article can beobtained by considering an approximate equivalent circuit for a junction transistor operating at small signals- Such an equivalent circuitis 7 shown for a ground base transistor in Figure 3.

'The circuit includes a grounded load R; which is conne'cted between'the collector and ground. The equivalent collector spreading resistance R and capacitance C are shown serially connected between the load R and the commonvjunction of the resistors r, and r the emitter vand base resistance respectively. The other side of the 'base resistance is grounded. The-voltages appearing at a the emitter and collector are given respectively byvgand v The various currents flowing in the transistor are given by i,,, i ,'and -i The alpha of the circuit is given i by brand the equivalent current generator in the collector is shown as cci a V If we assume that the transistor is being driven by a source'supplying an alternating current i then the effect of r;,, the base resistance, decreases the power gain by both decreasing the power output of the transistor for a I given value of i and also by increasing the power required to drive the amplifier. For example, even if the collector capacity C were zero, the base resistance r has a serious effect. For the model shown in Figure '3 with C equal to zero, the power output is fixed since the power output is notalteredby. increasing 'r l However, ;the

' collector capacity accents thedegradation due tor since :input ower required increases withincreasing r since anyiincrease in, r makes the capacitor a more attractive path for the current from the equivalent current generator ai in the collector. circuit and. correspondingly de- .thepower u 1 current densityLinto'i {of dimensions ndQdtjpm the V becomes comparableto unity. As pointed out by Lee in the reference cited below, the influence in a diffused base germanium transistor of C r and R may be such as to produce the major contribution to the observed alphacut-0E" and he introduces a frequency to describe this effect. It is thus evident that if f,, the cut-off frequency for the web, is substantially greater than f then there will be significant loss in gain below L. From theseconsiderations it is clear that reduction of r is desirable in junction transistors and that this is especially true at frequencies approaching f 'or f It is also evident that reduction of C and 'R is also desirable.

The spreading or bulk resistance of the collector body R may be reduced to'an arbitrarily small value by a variety of well-known techniques. For example, very heavily doped material may be used in the body of the collector and a lowly doped region formed near the surface by heat treating to diffuse'impurities. Alternately, the collector region may be made very thin. This need not be done over the entire volume of the specimen. For example, as shown in Figure 4, a dimple 18 may be cut into the specimen by jet electrolytic etching; then a very heavily doped layer 19 may be formed by diffusion, the top layer is polished away leaving a blank with weakly doped material backed up ata short distance by a highly conducting layer which constitutes the collector body.

The essential problem in reducing the C (R +r product, therefore, involves the reduction of the term C r or reducing the .valueof either of these constants. This invention, as'previously pointed. out, provides a structure and method for reducing the base resistance r In order more clearlyjto understand operation ofthe weband rib structure, the interplay-of several factors should be considered. 'This may be more clearly understood with reference toFiguresS and 6. Figure 5 shows a portion of one rib 17 and a portion of the working web 16 between the ribs. Figured shows the chemical charge density N -JSI along the lines A-Aand B-B of FigureS. 7

Referring particularly to Figure Q, it is noted that the chemical charge density rises to a much higher value in a the rib thaninthe ,webf-bet ween the ribs, thus leading to 60.

'afhighefconductivityin the 'ribsj This is achieved by heavily doping the ribareaand forming a web area which has'l'ess doping. The gradient or chemical charge density at the emitter and collector junctionsarefapproximately the same for the rib region as forpthe corresponding junctions of the web region." ,A structure having sentially these characteristics can beproduced by a number of, tecl1- niques, some of whichwill be presentlydescribed;

i The flowjv of injectedllcarrier f m'emitter into the base is much greater per unit area in the web region than in the rib region because the web. region-is both thinner and less -lieavily'dop edithan th 'bre'gio As is well-known, the I asevaries'approximatelyinversely ofac'torslf FBy proper selection as the product f th tors maybe. controlled As density in 'the rib is then about the web current density. The alpha cut-ofi frequency for the rib will be approximately ten times lower than that for the web so that: at frequencies approaching the alpha cut-ofi for the web. the current in the rib will be largely ineffective and the alpha of the transistor will be reduced. The reduction will not be great, alpha values of about 0.9 will result even for cases in which the rib width is approximately equal to the web width, as will be presently described.

Assuming, for purposes of discussion, that the emitter body is much more heavily doped than the web region and that it is comparatively thick, the emitter junction injection efiiciency approaches unity. For a thin web region, the fraction beta (,0) of injected carriers which 1 reach the collector junction will also be near unity so that, if all the base layer were a web, alpha would be near unity (in keeping with the quoted values of 0.98 or higher for diffused base transistors).

For a transistor having equal rib and web width, and operating at frequencies between the rib and the web alpha cut-off frequencies, the emitter current will contain approximately ten percent component of rib current for which alpha is nearly zero. Thus, the over-all alpha for the transistor will be the average value of. alpha for the rib and web regions or of zero plus 90% of 0.98 which equals 0.88. It is apparent that this leads to a reduction in power gain. Compared to the power gain 1 with an alpha of 0.98, the reduction is 19% in the fre quency range over. which" the gain is determined by the L. alpha of the web only. 1

Corresponding to the 10% reduction in alpha discussed above there will be a decrease in base resistance of approximately fivefold. This follows from the fact that -the conductance in the base layer parallel to the rib --direction arises chiefly from the ribs which have a conduc- "tance'in each unit of length approximately 10 times as 1 great as in the adjacent region of web, for the example in which rib and web widths are equal. If we compare :this with a transistor in which each rib is replaced by a :region of web then the two parallel strips of plate material will have a conductance for a unit length of two "units compared to a conductance of approximately 11 units for a rib and web in parallel. On the basis of this it would appear that the base resistance is reduced by approximately a factor of 5.5. This calculation neglects the fact that there is some resistance to current flow from the ribs to the central portion of the web between the ribs. The importance of this transverse flow in the rib structure will depend upon the base dimensions in any particluar casep The capacity of the collector junction will be approximately the same for the ribbed and unribbed structures 'since this capacity arises from thespace charge region 21 '(Figure 5) of the collector junction and dependsupon the concentration gradient of chemical charge density at the collector junction and'the collectorvoltage.

Thus, for the example discussedabove, the rib structure will reduce the product C r by approximately fivefold,

permitting operation'at five times the frequency so far as this limit is concerned. Alternately, the same C r, value 1 can obtain with larger dimensions withfthe rib structure,

thereby permitting higher power handling'capacities. V Summarizing, the: introduction of ribs may be used either toimprove the high frequency performance, or

' I tmincrease the dimensions which results in ease offabri- -,cation and'better power-handling capacity, or a-compro- .nnise of the two. i

The techniques forproducing layersdoped by donors or ithejn ovelfstructure of theinvention.

fabricatingthe structure is shown in Figure 7. A block diffusion constant on temperature and impurity type: are presented.

For the purposes of the following explanations the reader should be familiar'with the concept of the diffusion constant denoted by D and the diffusion distance which is given by the formula D! where t is the time of difiusion. If a source of impurities is maintained at the surface of a silicon crystal for a time t, for example by exposing the surface to a gas containing the impurity, then the concentration within the surface will build up to an equilibrium value C dependent on the gas pressure'and temperature, and the impurities will be distributed by diffusion to a depth below the surface in a way calculable from the laws of diffusion. The concentration drops rapidly to small fractions of C at depths greater than twice the diffusion distance and the majority of impurity atoms arefound within one diffusion length from the surface. It is evident that by controlling the nature and source strength of the impurity atoms, the temperature and the time, a large measure of control of the density of donors N and of acceptors N below the surface can be achieved. The mathematical theory found in' many treatises on the theory of heat and the -methods of theoretical physics. Curves for the distribu- -tions of donors and'acceptors based on surface concentrations and diffusion constants and times can readily be computed for specific cases for the structures presently described For this reason the curves shown in the figures are intended to illustrate important qualitative features rather than exact quantitative results.

Employment of these techniques for making diflused base transistors are well-known and have been described for germanium by C. A. Lee in an article A High Frequency Diffused Base Germanium Transistor, Bell System Technical Journal, January 1955, vol. 35, page 23, and for silicon by M. Tanenbaum and D. E. Thomas, Diffused Emitter and Base Transistors in the same journal on page 1.

The dimensions of typical transistor structures of the invention may be essentially the same as those discussed in these articles save that the dimensions of the base regions may be made larger for'similar high "frequency performance characteristics, as previously described. Thus the plate regions may be of the order of one micron ,(or' 1;() cm.) in thickness and donor and acceptor concentrations may be'of'the order of 10 to 10 cm.

.high values-by thisme ans. Such high densities are unstable and tend to lower their concentration by precipita .tion or diffusion. However, they may be preserved at' room temperature in the supersaturated condition. High er concentrationsmay also be produced by bombarding the surface by alpha particles or high energy electrons- (sayf2 million electron volts) so as to produce abnornial- -,ly high concentrationsof Vacant' sjlicon sites- These will H diffuse ,toth'e surfaceand willtendto draw inward irrjipurities on the surface.

, The diffusion technique lends itself nicely to, fabricating One 'meth'od bf I- acceptors bydiffusion have been extensively investigated"; "tfor"silicon and germanium. .-.The .datafor silicon is con-I veniently available in the article-Dilfusion'offDonor and 0 of semiconductor material 22 ,'jfo r .example n-type, is

grooved; on theto'p surface so as to prodhc'e an outfer profile'in cross section like that, shown in Figure fA'cceptor Elements in Silicon .byC. jSjFu'ller and I Any f a large numberoftechniques ay b jti e ll fDitaeriberger, Journal Of APPIi edf 'PhYSi CS, May, 19:56,. for grooving. "The groovesrnay becut w'ith 'averyifine yolutne 2 7fpage 544;--In-this article, the'dependence -of' -saw such as is usedyforslicing"crystalsof-silicon-orger asaasso manium into small wafers for use in making semiconductor devices. For the present purpose, of course, the fsaw cuts are made very shallow. Subsequent to the cutting, the surface may be etched so as to remove surface tective coating down inthe regions in which it is not desired to produce grooves and then etching the surface chemically or electrochemically through the spaces in the protective coating. The protective'coating may initial- -1y cover the. entire surface and the openings for the grooves may be made by removing strips of the protective coating, for example by making scratches through. the coating. It is evident that photoengraving techniques may: also be used. Evaporation of gold or SiO or other protective coatings may be used and the areas to be i etched may be shielded, for example by small wires.

'For the purpose of making extremely small grooves, it is possible to make use of electrochemical techniques combined with exceedingly fine wires known aswhiske'rs such as are discussed by S. S. Brenner, Acta Metallurgica, 'VOL'4, pages 62-74, January 1956. These whiskers have been observed in a number of cases and are smaller than -any actual drawn wires. A whisker used as an electrode "for-electroetching may thus be employed to produce extremely narrow electroetched grooves.

Whiskers of one metal may be coated with more chemically resistant metals. 1

Figure 7B illustrates'the structure produced on the grooved block by. dififusion in the presence of a high concentration acceptor impurity. Under the conditions,

. a layer of controllable thickness of strongly p-typematerial, denoted by.p(+) is produced below the surface.

. The resulting p-n junction follows the contours established by the grooves. Figure 7C illustrates the next step of the process in which the specimen is reduced in size by lapping, cutting withfla saw, or polishing. This operation 1 leaves a set of strongly p-type struts or ribs inserted in the surface of the block.

The specimen asprepared in Figure 7C is then exposed todiifusion in the presence of a high concentration donor ,impurity to form a strongly n-type layer denoted by n(+), which enters the surface as shown in Figure 7D. Simultaneously, thep-type impurities left in the ribs diffuse to, somewhat greater depths and also difiuse out of -the specimens' at the surface, thusreducing the concentration of p-typamaterial at the surface. i

' Inthecase of silicon, all of the acceptors from the third column of the periodic table, with the exception of boron and indium, diffuse-substantially faster. than the. donors. For example, aluminum at '1250 has a diffusion eonstant approximately 30 times larger than antimony. Becauseof this difference in diffusion constants, a subsequent diffusion in the presence of. an acceptor, at weaker concentrations than the donors of Figure7D, will result 3 in the idevelopmentof-a ,p-type layer lying under the .n(+) layer of Figure 7D. ,Figure 7B shows the struc- ..ture which resultswhenthe corners of Figure 7Djare 1bevelled'soas to remove the n(+) layer at the corners andth'e structure is' subjected to a subsequent diffusion 'n" the presence of an'accepto'r, forfexample' aluminum "at'12 50 in the case'of silicon. 'The're'sult is to develop,

"ap typ'e' base layerorweb connecting the p-type ribs of g j Figure 'l E. This produces the'desiredbase .lay'e r 'struc-' Zture' havingvarying thickness. Simultaneously," an area suitable for a base'con'tac-t is made since on'. the bevelled gnu-faces therewas no n(iI)-layera nd hence an exposed -p' -type region suitablefona base co ntact is formedf The jbevo r lif l 1 alliedges w y i T'rThus, ai specim'enofsilicon large enough to form many a. i d -fmay be'polished-and then in one continuous 75: qpgrqtigpf grooved over its entire area. The polishing j can be made to the ends, of the ribs. 1 f

It should be noted that after th diffusion discussed in i i cpnnectioniyvith Figure [E a slightifetch or polish maybe necessafyto produce the configuration shown in'Figure 7E. The reason for this is that due to gaseous and surface diffusion, the entire surface of the specimen tends :to acquire a uniform concentration of impurities in the f first, few layers of atoms; *Actually this does not always occur and the surfaceof aspecimen heated in a diffusion .1 furnacemay consist of n-type'and ptype regions. This failure to reach equilibrium with the outside environment is thought to be due to. rate limiting processes such as These rate limiting which is-depleted of donors at its surface by evaporation.

v This situation may be remedied by etching or polishing off the resulting p-type skin., Alternatively, a source of relatively slowly diffusing donors, such as arsenic or antimony for silicon, may be kept present so as to neutralize theacceptors on the surface, but not in depth. In this case an n-type skin will occur over the bevelled base contact regions of Figure. 7B. This skin may be removed by additional bevelling operations.

As is illustrated in Figure 7E, theremaining unwanted material is lapped or outer polished off along the line 2.3, and the exposed junctions areetched to improve their characteristics so as'to result 'ina semiconductor structure suitable for application of electrodes as illustrated in Fig ureJF.

, Themethod described lends itself to a technique for improving the characteristics of collector junctions. Fig- ,ures 7G and 7H illustrate the principles involved with some exaggeration of the possible bevel angles. A second bevelling operation 24 which gives a feather edge 26 to .the p layer at the collector junction is carried out. It

isevident that this feather edge willreduce the component of electric field along the surface when reverse voltages are applied across the oollector junction. This effect will permit operation at higher collector voltages, since as is well-known, the reverse breakdown in a silicon p-n junction is dependent both on concentration gradient at V Figure 7H repre sents theeffect of causing diffusion to occur after the bevelling operation of Figure 7G. The acceptors on the feather edge of the base layer difiuse in depth and thus reduce in concentration. *This results in a contraction 27 of the base-collector junction as represented in Figure 71-1. The concentration gradient across the junction itself is thus reduced near the surface. This configuration has the advantage over the situation of "7G of reducing col- .lector capacitance and the magnitude of the electric field inside the surface.

The same effect will occur at the base emitter junction 28 and thisfis also represented in Figure 7H.

Contactsrnay be made to thevarious regions as taught in'the references onfidiffu'sed base transistors referred to above. In the structures described, however, there is the advantage that thedarge'r area emitter-will facilitate making contact to'the emitter by making it easier to fevaporate a r'netal area electrode upon it. Also, the

- means proposed .for exposing the base layer may be replaced by alloying with aluminum through the emitter 7, layer to the p-type base layer in accordance with the method; discussed byTanenbaum and Lee in the cited reference. 1Also inthe case of p-type emitter layers, use

may be madeof the fact thatholes enhance electrolytic .etching rates, Thus iftheemitter isvmasked except for a desire'd area for base contaet, the base is reversebiased,

"land electrolytic etching employed, then'the exposed emitter layer may be etched away leavingthe base layer ready for contact.

jto production of diffused baseitransistors in quantity;

. 1 he-groovmg and bevellin g techniquesare well adapted assets-s2;

V, a may be taken by parallel lapping and polishing} fixture so that a large number of transistor structures arefabricated on the surface simtitaneously. The evaporation of emitter and base connection areas may take place through multiple openings in a mask for'the entire specimen. Subsequent to these operations the units may be cut apart by saws or magnetostrictive cutters and the individual Wafers mounted and packaged. It is also possible to carry out a soldering operation of all the collectors simultaneously to individual metal tabs prior to cutting the units apart. Figure 8 illustrates another method which may be employedlto produce a'transistor structure with a base hav- -ing]varying thicknesses. This method produces a-smal1er area interfacebetween the'ribs and col-lectonthus reducing the base collector capacity in comparison with the -'structureof Figure 7". Furthermore, a refinement of the procedure reduces the emitter base capacity in' the area of the-ribs. V a p Figure 8A represents a portion of the surface of 'a block "of material which has been exposed to diffusion of donors so as to build a relatively thick and: heavily doped n-type layer d'esignatedn(+) on the surface. This layer will subsequently compose the rib material. 'At this stage of the-operation the surface is plain, the grooves beingadded later." The: donor concentration duetodifiusion is represented in Figure 8D. This corresponds to the case of) a: constantsource strength during the diffusion process. For the purpose ofsubs'equently reducing therib-emitter capacity, anadditicmal diffusion is carried out in. Figure 8615 This is carried outfin the absence of a source of donors .so'th'at thedonors are losti'from the surfaceby diffusion: outwards.

'After this operation, the donor minus acceptor density in the. specimen is. approximately as shown inzFigure 8D, the region of highest concentration {beingapproxirnately a diifusionlength below the surface. :Thegnextoperationr consists of. formingsa set of grooves in the; surface.

These grooves will subsequently constitute the 'webareas of thetransistor; the spaces between the grooves constituting the ribs. After the'gr'ooves are prepared, a subsequent dilfusionIwith donorsiscarried "outt'; results in a thin n-type'. layer withza. high con- -centration near the surface, over the entire? surface of the specimen (Figure-8E). A cutxisthenmadeby.polishingdown thesurface. to adeptho-f theiorderco f aimicron to, remove this heavily doped n type layer. over the surface. 29', which is subsequently to becomerib material. ;The,resulting structure,is shown at FigureBFk. 'Iihecon- ,gentratiomgradients along the two. lines A andLBaway from the; surface; are, Shown-in Figure-.86; Polishing Qt l; the.- heavily. dopedvmtype; layer of high concentration gradient leaves a region of relatively. low concentration anddow concentration gradient over the plane surfaces.

. l Thenext' operation consists-of1allowing"acceptors to dnfusein from; the surface during a subsequent difiusion treatment.

This; results in a pen-p structure asshown in Figure 8H. Thep-layer over the rib materialisshown 'as substantialIy.thicker than: over theworking region since 1 in .the vneighborhood of the rib material the; concentratiqn ofd belowgthe, surface. Thusthe acceptors in this. regionzconvert the material to p-typerelatively'deeplytibelowrthe surface. On the other hand, over the workingregion the second. diffusion of donors produces a-highly concentratedbut thin layer of n-type material and. the p-n-. junction. isrelatively nearthe surface. The concentration gradient along fthetwo lines A: andxB. away from the surfacejs shown ats'L.

; ;-T he.- -a,dvantageof the? configuration shown: in' Figure 8-H... 1s that; a relatively low concentration gradient ,is pres :ent over'the ribs. Thisleadsfto a p n; junctionhaving a' low transition capacity. Oh'theother-handfitheactual onors builds up only "ata considerable: depth concentration of donors inthedepthfj theirib layer {high or higher j that j inthe region 'provided the surface conce ntratiorr during "the initial diffusion was substantially higher than-during the formation of the web region of the base. This will'result in a reduction of diffusioncapacity over the rib region. Furthermore, the area of collector junction per unit area of ribmay be made somewhat smaller in Figure 8 than in Figure 7 because the junction between the rib and the collector body is plane whereas in Figure 7 it is convex inwards. Thus, the structure of Figure 8 will have less capacity per unit area than a comparable structure produced as illustrated in Figure 7.

Still another means of producing conducting ribs is illustrated in Figure 9. In this case a rate-grown p, p+ structure is prepared from a silicon melt containing. both donors and acceptors. Since acceptors diffuse generally more rapidly than donors, a subsequent heat treatment in vacuum of this melt will lead to a loss of acceptors from the surface. The result of thiswill be to. produce compensated regions which are n-type. This is illustrated in Figure 9. These regions may then be used as ribs to reduce base resistance in a manner similar to that discussed above. Alternatively, the compensation may be produced or enhanced by causing donors todififuse in from the surface.

Figures 10-13 illustrate another form of rib structure junction separates the n(++) layer and the underlying block because of the light doping in the underlying block.

The heavily doped n( layer Will subsequently constitute the rib material. Next, grooves are cut in the structure, one of these being shown in Figure 10A.

Subsequent to the above operations, an n-type, layer which is to be used as the Web portion of the transistor base is formed by diffusion. The next two steps in the fabrication are illustrated in Figures 10B and 10C. ,These involve evaporation in vacuum (Figurell) of an insulating film and a metal film over the grooved structure. These. operations maybecarried out in a high vacuum bell'jar 31 using. sources 32,33 and 34. First .a-layerl of insulating material, such silica or magnesium fluoride, such asis evaporated from. sourcesf32 and.34"lonearlyinthe plane of the grooved surface so. that thejevaporating, molecules. strike the specimen at grazing incidence. Asrepresented inlFigure. 10B, thisresults in the formation of'an insulating layer I over measurface except at the bottoms of the grooves which lie in shadow with'respectto the sources of insulating material.

Subsequent to this operation, ,a metal layer is deposited fromtheusource 34' located approximately directly in fr'ont'ofthe face of the specimen. As illustrated in Figsuch as gold galliumare-suitable, metals to use for this purpose when silicon isthe semiconductor.

Thje resulting structure has high conductivity perpendic 11113.1" to the figure .for. both the emitter and the base region. In the case; of theernitter the conductivity is formed by the layerof aluminum which extends overthe entire surface For" the case of the base region theconductivity arises from the relatively thick and heavily doped 'ribs 37j The emitter base capacity is small .over the ribs "lie'cause'off the insulating layer I. o Q It should'be' notedthat the buildup of insulator on-the: sides of the grooves willtend to form 'projectirigridges.

which will shadow the sides of thegrooves in thesubseyquent'evaporation of metal. This eifect may be utilized Iin further reducing emitter base capacity and in improv- -Jing' base resistance. This desirable result may be accomplished bynallowing the ridges to build up to such a de- 315 'ggree that the evaporated metal forms in separate strips, Jone lying at the bottom of the grooves and the other on ;the regions above the ribs. After the alloying cycle has to'ccurred at the bottom of the grooves, the specimen may 'be'polished down so' as to expose the n( material at '10 the top of I the ribs. A subsequent evaporation of a ,metal film will then produce a metallic contact all along the ribs without producing a metallic path from the ribs to the emitter line. h v

The method of making contact to the device of Fig-:

ure 10 is illustrated in Figures 12 and 13. Figure ,12 represents a plane view and Figure 13 a cross section of the 'device. In this figure it is assumed that the evaporation of the insulating layer extends over an area much .larger than the area of the grooves between the ribs. fOn'the other hand the layer of metal is evaporated through 'a mask which restricts its, area to essentially that of the grooves. Subsequent to the evaporation and alloying, the plane top surface of the device is bevelled 38 by lap: pinfi, polishing, or etching or a combination of these so as to expose the rib material, n(++).- Contacts to ,this base layer may be made by subsequent evaporations and alloying or by masking and plating. Since the base layer is of very high conductivity, it is relatively easy to I, make low resistivity, substantially ohmic contacts to itr0 Contact to the collector may be made by plating or al- 1oying 39 to the base of the block and subsequent soldering to metal leads or massive metal supports 41; The ,contacts to the-evaporated emitter region M may be 'made byvpressure contacts, soldering or welding. If the 85 evaporated film is aluminum it may be advantageous to :make the metallic contact at the same stage that the alloying process is occurring; Under these conditions, it will be relatively easy to'penetrate the aluminum oxide layer which tends to form on aluminum. Alternatively, the layer of oxide may be vbroken by pressure combined with fine particles of abrasive or by puncturing the layer in the presence of molten solder with anelectric "spark of low energy A spark. will break throughthe oxide layer permitting the solder to wet the aluminum which 45 jwill then result in good metallic contact. i

v It is evident that although the methods disclosed 'above aresuited to the preparation of relatively large area junc- 'tion transistors, their value is not restricted to such cases. :Thus' the fabrication of relatively small area junction transistors can be substantially simplified by the use of the frib'principle. An example of the use of the rib principle 'to construct a small area transistor is illustrated'in Fig- 'ures1416.' f .The structure shown'in Figure. 14' is fabricated in a .55 .manner similar to that of Figure 8; Since the individual steps are similar to those of Figure 8 theyjare not'represented in the diagram. As a first-step, the rib structure is produced by diffusing ap-type' impurity, ?for example .boron and silicon, onto asmoothly polished n-type blank. '(For' purposes of illustrating the flexibilityof the .t echnique, the polarity is made opposite to that of Figure'iS.) After this diffusion an emitter area is cut intothe surface. The emitter area 40 consists of tworegions Aland 42, region 4lrwhich is used, for. making contact to the emitter, and working region 42. 'Theiworking'region '42 is a'fdcep groove.' The contact reg on? issh'allower. Next donors and acceptors are'diftusedso and form an n-type surface'flwith amore, deeply penetrating ptype 'layer, Forrexample, aluminum and antimonymaylbe ,70 .used since aluminumdiffuses more rapidlyftlian antimony and the structure will result" if both are diffused simultaneously. Subsequent to thisoperation,theentirespecimen contains an n-type skin dueito'theantimoiiyLr n a te? enswear. t eis rt esf it.

thespecimen so asto remove the n-type skin from the rib .of the baseregion. Alternatively the grooves maybe filled with an etch resisting material, such as a wax, excess wax removed from the 'tops of the ribs and the n-layer etched off the tops-of the ribs.

Next cuts 43 are made either mechanically by lapping, or chemically by masking and etching so as to limit the extent of the rib of the base region. The resulting structure is shown in section, Figures 15 and 16. Referring to Figure 15, we see that the p(+) layer has been removed from the area over the 11b. The n(+) layer lies ..at asufiiciently'shallow depth in respect to the rib material so that it makes a' relatively low capacity junction "with the rib material since near the surface the rib materialhasv been depleted by evaporation from the surface, as was discussed in connection with Figure 8D. I

- ;Also shownin Figure 15 *is an alloy contact made to the n(+) layer.- Since'in'the shallow region the n(+) .layer lies upon a thick region of p-type material which is not very heavily doped at the p-n junction, an alloy contact'can be easily made to it with a gold antimony alloy or other donor bearing alloy.

In Figure 16, the deeper portion of the groove con- "taining the web or active area of the base is shown. Also shown is the base contact which is made to' the p-type layer by alloying with an aluminum wire or an aluminum coated wire, for example. I

, The advantages of the rib technique in making junction transistors is apparent from Figures 14-16. It is seen that by the utilization of this technique, the problems of contacting the emitter and'the base region have been materially simplified 'andthe problem of geometrical control does not involve'the evaporation of a metal area 'of controlled shape but is reduced to controlled etching of cutting. These operations can be carried out with a .pre'cisioncomparable to that involved in making ruled gratings which is quite adequate for making transistors of hitherto unattained performance. It is apparent that the 'method disclosedwith reference to Figures 14-16 can be utilized for'fabricating large area transisors of the type lhereinbe'fore described.

The basestructure' of the" invention need not consist of T a series of parallelribs only, but it is also practical to produce structures in which a set of heavy ribs run in one direction and .arec'onnected with a set of more closely spaced, somewhat lighter ribs running at right angles .tliereto: .Theimain current path from a representative area of web totthe base "contact will be from the web to "one' of the narrow ribs and along the narrow rib to one of theheavier 'ribs'and thence tothe base electrode. Such 'structures maybe particularly advantageous in large area high powered transistors. V I r r :It is evident from the foregoing description that in ceritain instances the web and rib regions may be formed of uniform thickness with the rib region having asubstanrtiallyhigher carrier concentration toincrease the conductivity o'f th'e' rib region or; the base layer; a

7 Although "the :description in general haskbeen directed -to'siliconand 'germanium semi-conductive devices, the inventiori is not intended to belimited in this respect-since other suitable semiconductors may be employed to form the'junction transi'stor of-theinvention. i7? Iclaim:"-""' 'i 1 1 -1 A 'junction transistor including emitter, base and collector, regions, and ohmic emitter, base and collector 'contacts thereto respectively, said base region comprising :relatively lowfresistance rib portions and'relatively high resistance web portions, said rib portions being in contact with the ohmic; contact, said base ohmic contact being .made 10 8.12} leash-one; end of the ribs whereby the average resistanceg from web to ,contact :is substantially lessthan the avera'ge. resistance fromiweb-to contactlin a-base in emitter region extending over at least a' orti "rhade'with at least one end of thelelongated por of; ltlhe thick elongated portions to fcrnifa junctionthere- 3. A junction transistor including "emittenbase and collector regions, and connections thereto, said base region being formed with elongated rib portions and web portions, said base connection being made to at least one end of the elongated rib, said elongated rib portions forming a low resistance path to the base connection.

4. -A junction transistor including emitter, base and collector regions, and connections thereto, said base region comprising a web portion which is adapted to operate at relatively high frequencies, and a rib portion forming a low resistance path, said base connection being made to at least one end of the rib portion, said emitter region extending over a portion of the rib portion and forming a junction therewith.

5. A junction transistor comprising a first zone of semiconductive material of one conductivity type, said first zone being formed as a web and rib structure, a pair of zones of semi-conductive material of opposite conductivity type contiguous with and on opposite sides of said zone to form a continuous junction therewith and means providing electrical connections to each of said zones, said connection. to said first zone being at least with the ends of the ribs. j

6. Apparatus as in claim wherein said ribs have a relatively high carrier concentration.

7. Apparatus as in claim 5 wherein said web is relatively thin whereby the transistor is operable at relatively high frequencies, and wherein said ribs are relatively thick to reduce the resistance to the base connection.

8. A junction transistor comprising a first zone of semiconductive material of one conductivity type and a pair of zones of semi-conductive material of opposite conductivity type contiguous with and on opposite sides of said first zone, ohmic contacts formed with said zones,

, said first zone comprising relatively low resistance rib portions and relatively high resistance web portions, said rib portions being in contact with the ohmic contact, whereby the average resistance of the first zone from web to contact is substantially less than the average resistance from web to contact in a zone in which the rib portion is replaced by web. I

9. A junction transistor comprising emitter, base and collector zones, said base zone being formed as a web and rib structure, said rib structure comprising relatively thick ribs formed in a longitudinal direction and thinner ribsformed at right angles thereto whereby the main current path from web to rib will be from the thin to the thicker rib. I

10. Apparatus as in claim 9 wherein said'rib portions have a relatively high carrier concentration.

11. A junction transistor comprising emitter, base and collector zones with said zones forming a pair of junctions, said base zone including rib and web portions, said emiteonecme regions forming emitter; base; and base semester unctions, said base region including web and-ribpo'rtior'is withthegradient iof chemical-charge density of thebasecollector; junction being less atthe" ribs than at the 'web.

16;A junction transistor including emitter, base and collector regionsf orming-emitter base and base collector junctions, cnnectionsformed1with said regions, said base. region including rib and web portions with'the rib portion in contact with the base connection, whereby the average resistance from web to connection is substantially less than the average resistance from web to connection if the rib portion is replaced by web, and wherein the gradient of chemical charge density at the base collector region is less at the rib portions than at the web portions.

17. A junction transistor including emitter, base and collector regions forming emitter base and base collector junctions, said base having relatively thick and relatively, thin portions, a region forming an emitter base junction in which the chemical charge density is relatively low near the junction over a distance relatively large compared to the thin portion whereby an emitter contact may be easily made.

18. A junction transistor including emitter, base and collector regions, said base region comprising working web portions and rib portions having relatively high chemical charge densities providing a low resistance current. path, said emitter region extending over the web portion and at least a portion of the rib portions and forming a junction therewith, and a base connection made with at least one end of the rib portions.

19. A junction transistor comprising a first zone of semiconductive material of one conductivity type, a second zone of semiconductive material of opposite conductivity type forming a junction therewith, said second zone having relatively thin elongated portionsand relatively thick elongated portions, an insulating layer formed over said relatively thickportions, a metallic layer disposed over said insulating layer and forming a rectifying junction with the relatively thin portions, means forming ohmic connections with the relatively thick portions of said zone and means forming ohmic connection with the first zone.

20. A junction transistor as in claim 19 in which said ter base junction extending only over a portion of the rib area. i

12. A junction transistor comprising emitter, base and collector zones, said base zone being formed by sohd diffusion and including web and rib portions.

13. A junction transistor including a semi-conductive body having emitter, base and collector zones forming emitter base and base collector junctions, said base comprising a n'b -.and web structure, the chemical charge density being substantiallysmaller ,where the base collector junction approaches the surface of the body.

' 14. A junction transistor as in claim 13 in whichth'e 5 gradient chemical charge. density of the base-enutter unci ,tionis'substantially smallerat the surface of the body..

415. junction transistor including emitter, base and trode.

charge density.

21. A junction transistor including a first region of semiconductive material of one conductivity type, a second region of semiconductive material of opposite conductivity type forming a junction therewith, said second region including portions of relatively high chemical charge density, a groove formed in said second region, said groove having relatively deep portions and relatively shallow portions, a third region of semiconductive material of said first conductivity forming a junction with the second region in the groove, means forming ohmic contact with said third region in the shallow portion of the groove, means forming ohmic contact with the second.

region, and means forming ohmic contact with the first region. a

22. A junction transistor comprising a'middle zone having ribs formed on one surface thereof whereby the zone includes rib and web portions, a zone of opposite conductivity type provided on said one surface and forming a junction with the rib and web portions, and another zone of opposite conductivity type provided on the oppositesurface of said middle zone, said zones of opposite conductivity type being provided with ohmic electrodes and said middle zone being provided with an ohmic elec- 2.3. A junction transistor claim 22 in which the ribs have a relatively low resistance. compared with other portionsof the middle zone.

24. -A junction transistor as in claim 22 in'which the 1 gradient of impurity atoms at at leastpne innetiqn isles:

at the rib than at the web.

"25. A junetiontransistor as in claim 22 inwhich'the web is adapted to operate at relatively high frequencies.

26. A junction transistor as in claim 22 in which there is additionally provided thinner ribs .formingan angle with said ribs whereby the current path from web torib will be from the thinner ribs to said rib portions.

16 References Cited in the fileof this patent UNITED-STATES PATENTS v 2,858,482 Henkels Ogt. 28, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. $980,830 April 18' 1961 William Shockley ified that error appears in the above numbered pat It is hereby cert he said Letters Patent. should read as ent requiring correction and that t corrected below.

Column 11, lines 24 and 25 for "lappinfi" read lapping Signed and sealed this 28th day of November 1961.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer V DAVID L. LADD Commissioner of Patents USCOMM-DC- 

